Recently, in the field of storage systems, a concept to operate multiple dispersed computing resources virtually as a single resource is attracting attention. One form for realizing such system is a configuration where multiple storage subsystems are collectively gathered and handled as a single system. For example, Patent Literature 1 teaches a configuration in which a storage subsystem is connected to a host computer and an external connection storage subsystem (hereinafter referred to as external storage) is further connected to the storage subsystem, wherein the whole configuration is considered as a single system.
This technique relates to connecting an external storage subsystem to a migration source storage subsystem (old model), and migrating volumes of the external storage subsystem to a migration destination storage subsystem (new model). Then, during migration, when the migration source storage subsystem receives a write request from the host computer, the migration source storage subsystem transfers the write data directly to the external storage subsystem without storing the data in the cache of the migration source storage subsystem. Further, when the migration destination storage subsystem receives a write request from the host computer, the write request is transferred to the migration source storage subsystem. As described, data is migrated from the old model storage subsystem to the new model storage subsystem including an external connection volume provided by the external storage subsystem without having to stop data transfer performed in response to an I/O request from the host computer.